// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package context_test
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import (
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"context"
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"fmt"
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"time"
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)
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// This example demonstrates the use of a cancelable context to prevent a
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// goroutine leak. By the end of the example function, the goroutine started
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// by gen will return without leaking.
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func ExampleWithCancel() {
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// gen generates integers in a separate goroutine and
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// sends them to the returned channel.
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// The callers of gen need to cancel the context once
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// they are done consuming generated integers not to leak
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// the internal goroutine started by gen.
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gen := func(ctx context.Context) <-chan int {
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dst := make(chan int)
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n := 1
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go func() {
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for {
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select {
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case <-ctx.Done():
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return // returning not to leak the goroutine
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case dst <- n:
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n++
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}
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}
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}()
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return dst
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}
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ctx, cancel := context.WithCancel(context.Background())
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defer cancel() // cancel when we are finished consuming integers
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for n := range gen(ctx) {
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fmt.Println(n)
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if n == 5 {
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break
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}
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}
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// Output:
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// 1
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// 2
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// 3
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// 4
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// 5
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}
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// This example passes a context with an arbitrary deadline to tell a blocking
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// function that it should abandon its work as soon as it gets to it.
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func ExampleWithDeadline() {
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d := time.Now().Add(50 * time.Millisecond)
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ctx, cancel := context.WithDeadline(context.Background(), d)
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// Even though ctx will be expired, it is good practice to call its
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// cancelation function in any case. Failure to do so may keep the
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// context and its parent alive longer than necessary.
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defer cancel()
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select {
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case <-time.After(1 * time.Second):
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fmt.Println("overslept")
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case <-ctx.Done():
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fmt.Println(ctx.Err())
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}
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// Output:
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// context deadline exceeded
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}
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// This example passes a context with a timeout to tell a blocking function that
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// it should abandon its work after the timeout elapses.
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func ExampleWithTimeout() {
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// Pass a context with a timeout to tell a blocking function that it
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// should abandon its work after the timeout elapses.
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ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
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defer cancel()
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select {
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case <-time.After(1 * time.Second):
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fmt.Println("overslept")
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case <-ctx.Done():
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fmt.Println(ctx.Err()) // prints "context deadline exceeded"
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}
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// Output:
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// context deadline exceeded
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}
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// This example demonstrates how a value can be passed to the context
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// and also how to retrieve it if it exists.
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func ExampleWithValue() {
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type favContextKey string
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f := func(ctx context.Context, k favContextKey) {
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if v := ctx.Value(k); v != nil {
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fmt.Println("found value:", v)
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return
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}
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fmt.Println("key not found:", k)
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}
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k := favContextKey("language")
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ctx := context.WithValue(context.Background(), k, "Go")
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f(ctx, k)
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f(ctx, favContextKey("color"))
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// Output:
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// found value: Go
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// key not found: color
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}
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